1. Field of the Invention
The present invention relates to a method for removing sulfate ions from an aqueous solution of an alkali metal chloride.
2. Description of the Prior Art
When an aqueous solution of an alkali metal hydroxide, chlorine and hydrogen are manufactured by electrolyzing an aqueous solution of an alkali metal chloride (hereinafter called "brine"), it is necessary to remove sulfate ions which penetrate into the brine system mainly from the alkali metal chloride used as a raw material.
As methods for removing sulfate ions from the brine, a barium salt method, a calcium salt method, a freezing method, a brine purge method, etc. are known, but these have the following disadvantages. That is, in the case of the barium salt method, barium chloride, barium carbonate, etc. used as additives are toxic and also expensive, in the cases of the calcium salt method and the freezing method, the removal rate is lowered when the concentration of sulfate ions in the brine is desirably controlled at a low level, thus resulting in increasing cost, and in the case of the brine purge method, a loss of alkali metal chloride increases when the concentration of sulfate ions in the brine is desirably controlled at a low level, resulting in increasing cost.
Recently, as a method displacing these methods, there is known a sulfate ion adsorption method, which is disclosed in, for example, Japanese Laid-Open Patent Publication No. 44056/85 and Japanese Laid-Open Patent Publication No. 228691/85). These methods, however, have the following disadvantages.
The method disclosed in Japanese Laid-Open Patent Publication No. 44056/85 consists in removing sulfate ions from brine by a macroporous cation exchange resin composite having polymeric zirconium hydrous oxide in a vessel. In this method, water is used for regeneration of the polymeric zirconium hydrous oxide adsorbing sulfate ions, as described by Examples 1 to 3, but it is apparent that this method is by no means economical because the regeneration efficiency is low and a large amount of the expensive cation exchange resin is required for carrying polymeric zirconium hydrous oxide. Furthermore, in this method, the polymeric zirconium hydrous oxide adsorbing sulfate ions comes into contact with acidic brine containing sulfate ions, hence loss of the polymeric zirconium hydrous oxide due to acid-induced dissolution of the polymeric zirconium hydrous oxide is caused to result in increasing cost, and the dissolved zirconyl ions precipitate again in the form of a hydroxide in the lower portion of the vessel to thus clog a flow path, and therefore the method can not be applicable stably or economically.
Meanwhile, disclosed in Japanese Laid-Open Patent Publication No. 228691/85 is the method in which brine containing sulfate ions is diluted to an alkali metal chloride content of not more than 120 g/l to cause the sulfate ions to be adsorbed to anion exchange resin and the anion exchange resin adsorbing sulfate ions is caused to be regenerated in an aqueous solution of an alkali metal chloride with a concentration of not less than 280 g/l. As described in the specification of the aforementioned publication, however, this method consists in adding a concentration process by the ion exchange method to the known sulfate ion removing technique, hence it has a disadvantage of increasing cost as compared with conventional methods.